Benitez had hoped to push Manchester City for second place but the Blues now find themselves looking over their shoulders after they were beaten 2-0 by Roberto Mancini's team.
Tottenham could leapfrog Chelsea into third by beating West Ham on Monday night and Arsenal trail Benitez's team by only two points.
"I said before the game, [if we win] it will still be a battle to the end," Benitez told Sky Sports. "Not winning is exactly the same situation. We have to carry on until the end."
Goals from Yaya Toure and Carlos Tevez sealed Chelsea's defeat after Joe Hart saved a Frank Lampard penalty, denying the midfielder his 200th goal for the Stamford Bridge club.
Benitez admitted that the penalty save in the 50th minute was a turning point despite his team's unspectacular performance until that point.
"The penalty is a crucial moment, we did not do as well as we were expecting but the penalty could have changed everything," he reflected.
"It was a good penalty and a good save. 10 minutes later, they score. Then we have to take a risk and it is clear they are a good team."